This invention relates to a method of quenching a hot metal object, particularly one made of steel.
The thermal quenching of hot metal objects is a required step in many heat treatment processes such as, for example, annealing, hardening, case hardening, carburising, or nitro-carburisation of steel objects. Typically the metal object is cooled by thermal quenching from a temperature of 850xc2x0 C. or above to a temperature of less than 100xc2x0 C.
Water has been used as a thermal quenchant, but by itself, provides too rapid a quenching in most examples of the heat treatment of engineering steels with the result that distortion of the object or internal damage to it is caused.
It is therefore far more common to employ an oil as the thermal quenchant in a heat treatment process. Several disadvantages arise from such use of an oil. Prime among these is that oil would pollute the environment and therefore presents a disposal problem when it is no longer fit for further use. Further, oils tend to give rise to noxious fumes and can present a fire hazard. In addition, the oil needs to be washed off the metal object at the end of the thermal quenching step.
Attempts have therefore been made to find alternative thermal quenchants to water alone and to oils.
It is known, for example, to dissolve water soluble organic substances such as a polyvinyl alcohol, an alkylene glycol, or glycerol in an aqueous thermal quenchant so as to reduce the intensity of the quench. Although such materials are non-flammable and do not give off any fumes in use, they still represent an end-of-life disposal problem.
GB-A-986 756 relates to the cooling of a hot solid body in a fluidised bed, that is a moving bed, of solid metallic particles. The bed is fluidised by means of a stream of liquid, typically water, at ambient temperature. The fluidised particles disrupt an insulating vapour film that forms around the body to be cooled and therefore enhances the cooling rate. Other documents also disclose the cooling of hot articles by means of a fluidised bed of particles. One example of such a document is JP-A-306 4421 which discloses the cooling of hot steel wire rod by means of a fluidised bed of metal particles. Another example is WO-A-00/17405 which discloses the cooling of steel wire by means of a fluidised bed of oxide particles.
JP-A-1100 217 discloses a quenching agent consisting of water, polyethylene glycol, and colloidal silica. The quenching agent shortens the vapour film stage during the formation of martensite in a steel article.
U.S. Pat. No. 4,243,439 employs a quenching medium comprising coaqueous suspension of a binder and a pulverulent filler component selectively to modify the density, viscosity and heat conductivity of the medium for use in the quenching of aluminium alloys from 525xc2x0 C. According to the teaching of U.S. Pat. No. 4,243,439, the presence of the solid suspended particles opposes the establishment or the stabilisation of an insulating calefaction film on the surface of the castings.
U.S. Pat. No. 5,681,407 discloses a method of quenching a wrought metal object formed of aluminium, iron, magnesium or an alloy thereof in which a liquid quenchant (typically water) is employed. The liquid quenchant has a gas such as carbon dioxide deliberately pre-dissolved in it. The gas does not cause any disposal problem, and does have some effect in lowering the quench rate. Nonetheless, this method still seems unsatisfactory for many engineering steels in that the maximum cooling rate is too high, as is the cooling rate at the temperature (about 300xc2x0 C.) at which martensite forms.
There are two problems which need to be solved. First, there is a need for a thermal quenching medium (xe2x80x9ca quenchantxe2x80x9d) which gives the operator some degree of control over the cooling rate for a given quenchant temperature. Second, in the case of engineering and other steels, there is the need to find a quenchant which makes possible lower cooling rates, particularly at temperatures in the order of 300xc2x0 C., without giving rise to any serious disposal problem.
It is therefore an aim of the invention to address these problems.
According to the present invention there is provided a method of quenching a hot metal object formed of steel, the method including immersing the hot metal object in a suspension of an essentially insoluble inorganic particulate material in water, the suspension being initially at a temperature below 100xc2x0 C.
The method according to the invention is suitable for treatment of alloys such as engineering steels that undergo an austenite-martensite transition during quenching or otherwise require a relatively slow cooling rate. In addition, the method is particularly suitable for treatment of high alloyed steels or tool steels which do not require a fast initial cooling rate and which would crack if cooled too quickly. Examples of such steels are molybdenum or tungsten high speed tool steels. The quenchant also has the advantage of not causing any substantial disposal problems.